Chlorine dioxide is a strong, but highly selective oxidizer. It can be used in aqueous solution for disinfecting drinking water and in other water processing applications. One of its chief benefits is that it does not react with organic materials to form chlorinated hydrocarbons which have been shown to cause health problems. In fact, chlorine dioxide not only destroys organic compounds but also destroys hazardous chlorinated hydrocarbons, once formed. Solutions of chlorine dioxide can also be used as a disinfectant for the decontamination of bio-contaminated buildings, enclosures, and articles and as a disinfecting wash for poultry, beef, and many types of fruits and vegetables.
Chlorine dioxide can also be used in the gas phase as a disinfectant. The gas can be used to sterilize medical instruments and other medical articles, as described in U.S. Pat. No. 4,681,739. The gas has been used to decontaminate buildings containing Anthrax spores after the Anthrax attacks of 2001. The gas can also be used for decontamination of buildings infested with mold and as a decontaminant for bio-safety cabinets and other laboratory enclosures.
Liquids called “Stabilized Chlorine Dioxide,” “Chlorine Dioxide Solution” and the like have been commercially available in the past but these liquids do not contain chlorine dioxide; rather, they contain sodium chlorite. When mixed with acid sodium chlorite reacts to produce chlorine dioxide, but this requires chemical mixing and handling of acid. Opportunities abound for errors in mixing and, even when reagents are mixed properly, the resulting solution contains high levels of salts, acids, and other impurities. Moreover, such chlorine dioxide solutions have a short shelf life once they are mixed.
Chlorine dioxide can be produced in a variety of ways. Most of the production processes that are suitable for less than a few thousand pounds per day are based on reaction of sodium chlorite with chlorine or acid in an aqueous solution. Many of these processes are based on the reaction:2NaClO2+Cl2=>2ClO2+2NaCl  Reaction 1Which can be carried out in an aqueous solution, or in the absence of water in what is known as the Gas:Solid™ ClO2 process.
Other production processes involve mixing dissolved sodium chlorite and hydrochloric acid, (Reaction 2), or mixing dissolved sodium chlorite with hydrochloric acid and sodium hypochlorite solution (Reaction 3).5NaClO2(aq)+4HCl(aq)=>4ClO2(g)+2H2O+5NaCl(aq)  Reaction 22NaClO2(aq)+HCl(aq)+HOCl(aq)=>2ClO2(g)+2NaCl(aq)+H2O  Reaction 3where (aq) refers to an aqueous solution and (g) refers to both evolved and dissolved gas
There are several drawbacks to these production methods. First, the products are impure. In addition to chlorine dioxide, product solutions also contain the other products and by-products of the reaction and unreacted feedstock reagents. Typical contaminants in these products include hydrochloric acid, sodium hypochlorite, sodium chlorite, sodium chlorate, chlorine and sodium chloride. In addition, the reactions are exothermic and produce substantial amounts of heat.
In response to the Anthrax attacks of 2001, several buildings were decontaminated using chlorine dioxide gas produced in solution using Reaction 3. The production of chlorine dioxide gas for the decontamination required pumping product solution of Reaction 3 through packed columns and blowing air over the solution to strip the dissolved chlorine dioxide from the solution into the air. The gas was then blown into the space to carry out decontamination. Despite its complexity, this process was very effective at killing Anthrax spores. Nevertheless, several reported problems with this method reduced its effectiveness.
As chlorine dioxide gas is stripped out of solution in this method, micro-droplets of solution became entrained in the resulting gas stream and were carried into the building. These droplets deposited a corrosive mixture of water and contaminants throughout the building space causing severe corrosion and other damage. Even where there was no corrosion, salt deposits had to be removed after decontamination was complete. In addition, the solution became hot as it was produced and when pumped into the stripper column (material transfer device) and contacted with air heated by the blowers, the gas stream was warm and humid. This resulted in severe condensation within the building when the warm, humid air contacted cooler air and cooler surfaces in the building. Large mechanical cooling units were used to partially dehumidify the air but a method was never found to control humidity and chlorine dioxide gas concentration simultaneously.
Thus, new methods are needed for decontaminating surfaces and enclosures that avoid problems such as salt deposits corrosion and condensation that accompany the use of previously known methods.